1. Field of the Invention
The present invention relates to image processing, and more specifically to systems and methods for displaying with automatic reframing a video on a display screen such as a television screen.
2. Description of Related Art
The available display screens exhibit a predetermined screen ratio (also referred to as the video format or aspect ratio or aspect format), that is to say the width/height ratio of an image. For example, standard televisions have a screen ratio of 1.33:1 (4/3 or standardized format), that is to say they are 1.33 times as wide as they are high. Likewise, widescreen televisions have a screen ratio of 1.78:1 (16/9 or widescreen format). Certain televisions may effect the display of images selectively according to one or the other of these formats, or even according to other formats (in particular, for some, a 14/9 format). Moreover, certain more recent display devices, such as LCD (“Liquid Crystal Display”) or plasma screens, exhibit screen ratios of still different values.
Aside from this, the video images to be displayed have a screen ratio which may vary depending on the origin of the video. Thus, videos obtained via mass-market video cameras such as camcorders generally have a 4/3 format. On the other hand, films produced for television typically have a 16/9 format. Furthermore, cinema films can have a screen ratio of 2.35:1 (Cinemascope format) or of 1.85:1 (Panavision format, Vistavision format, etc.), for example.
Video broadcasting as standardized according to the recommendations of the ITU (International Telecommunication Union), formerly the CCIR (Comité Consultatif International des Radiocommunications), involves a 4/3 format both for images having 525 lines and for those having 625 lines. A cinema film, for example, is therefore processed according to a “Letterbox” technique which adds black bands at the top and bottom of the original images (without modifying them), so that the images broadcast are displayed without distortion on a standard television (4/3 format).
FIG. 1 shows an example of an image thus processed so as to allow broadcast according to the aforesaid recommendations of the ITU. Such an image is said to be in the “Letterbox” format. In the example shown, the outer box 1 corresponds to the 4/3 format image, and the inner box 2 corresponds to the 16/9 format image, that is to say to the active video. The difference between these two images resides in the upper horizontal black band 3 and lower horizontal black band 4.
The need exists for a process that makes it possible to automatically detect the format of a video to be displayed, so as to adjust the display parameters in such a way as to optimize the display. Such a process could be used to implement a reframing of the images received in the “Letterbox” format, for example a zoom, so that the active video is displayed in full screen mode on a 4/3 format screen, that is to say without having horizontal black bands appear.
For this purpose, it is known to detect the upper and lower black lines of the video image, according to various methods that have been proposed. This amounts to detecting the active video within the video.
These known methods nevertheless exhibit insufficient accuracy, in the sense that the detection of the black lines may be deficient in a certain number of situations, depending on the content of the images. Moreover, the known methods exhibit limitations in terms of flexibility (e.g., no possibility of changes) as well as a higher cost when they are implemented in hardware.